1. Field of the Invention
This invention relates to valves and more particularly, to a valve to improve the performance of a bushing seal surrounding a rotary shaft.
2. Description of the Prior Art
As shown in my U.S. Pat. Nos. 3,724,861 and 3,679,217, bushing seals are well known for sealing a rotary member such as a shaft or an impeller of a pump or compressor. Such bushing seals are used to seal a pressurized fluid contained, for example, within the interior of a compressor from a low pressure area such as ambient atmosphere. As shown in the above-mentioned U.S. patents, the bushing seal comprises a bushing surrounding an impeller with opposing surfaces of the bushing and the impeller defining a restricted fluid passageway between a high pressure area (such as the interior of the compressor) and a low pressure area (such as the ambient atmosphere). A sealing liquid is supplied to a pressure chamber defined between opposing surfaces of a housing surrounding the bushing and the bushing itself. The sealing liquid is supplied under pressure from the pressure chamber to the restricted fluid passage intermediate the low and high pressure areas. The fluid is supplied at sufficiently high pressures to effectively block a flow of process fluid within the compressor from the high pressure area toward the low pressure area. A face portion of the bushing abuts the housing surrounding the bushing and shaft and is urged against the housing by reason of the pressure of the sealing fluid to form a secondary seal. Bushing seals, as described above and shown in the aforementioned U.S. patents, are free floating within the potential movement of the shaft and restricted in movement by the frictional forces between the face portion of the bushing and a bushing housing (O-rings sealing the bushing from the housing provide only a negligible resistance to motion of the bushing).
Currently, pumps and compressors are being designed for applications having a wide range of pressures of contained fluids. At very high pressures, the sealing fluid is necessarily supplied at high pressure and, as a result, the bushing face portion may be urged against the housing at such a great force that the bushing essentially freezes and acts as an undesired bearing for the shaft. Accordingly, for pumps and compressors to be operated at high pressures, the face portion of the bushing is designed to have a sufficiently small effective area so the bushing does not freeze at the design operating pressures. However, when the compressor is operated at low pressures, there is little friction between the bushing and the housing (due to the small effective surface area of the face portion and necessary drop in sealing liquid pressure) resulting in the bushing becoming unstable and developing what is known as a subsynchronous whirl which may result in failure of the compressor shaft.
I have determined the phenomena of subsynchronous whirl may be avoided at low operating pressures by providing a valve which operates when the pump is operating at low pressures to generate an additional force urging the bushing face against the housing.